a fighter's fighter: the Curtiss P-40 was successful in nearly every WW2 operational theatre except Western Europe. However, quite few aces attained high scores in it. The following are the ten most successful P-40 aces that I know, ranked by their P-40 scores (totals in brackets):

Caldwell: 20,5(28,5)

Novichkov: 19(29)

Drake: at least 16(21)

Pokryshev: at least 15,1(38+8sh?)

Hill: 13,25(13,25)

Hampshire: 13(13)

Holloway: 13(13)

Neale: 13(13)

Edwards: 12,83(16,08)

Waddy: 12,5(15,5)

Compiling them I came across a guy named Robert Lee Scott whose score is unclear to me. Does he deserve a rank in this list ?

George Mellinger in "Sovie Lend-Lease Fighter Aces of World War 2" assigns a P-40 score of 19 and a total of 29 to Stepan Novichkov.
However, this websitehttp://soviet-aces-1936-53.ru/abc/n/novichkv_s.htm
gives him a P-40 score of 6,33 and a total of 33,33.
Which is more valid ?

I know it has been pointed out to you before, but you really do need to obtain the Mikhail Bykov book, Все асы Сталина 1936–1953 (All Stalin's Aces). I obtained a copy cheaply and quickly from a bookseller in Moscow, Viktor Kulikov: viktor_kulikov@mail.ru.

Although entirely in Russian, it is easy enough to follow, and Google Translate is very handy.

This will answer all of your questions on Soviet aces. Books like those by Mellinger are superseded entirely by the work of Bykov.

IMHO Bykov is the man, he has went through the archives, the western writers had only used secondary Soviet era sources and are unreliable. I have Polak's book and it was disappointment. I doubt that Seidl's book is any better.